You can keep track of your data use on your phone via Settings>Cellular. You just need to remember the reset it at the beginning of your monthly cycle.

You can also track your data usage on your self serve app on your phone or you can login via webpage. Just note that there is usually a little bit of delay as the data usuage seems to update only a couple of times a day.

When you notice that you are nearing you data limit you can turn off data at that point.
You can also to make sure you use WIFI whenever it is available (home/school/work) so you can minimize your data use so you dont go over

If you don't really use data at all, You can go to settings>cellular>scroll down and disable whichever apps you want from using data. By doing this, for the occasional time you turn on data to send mms, your apps won't munch off of your data. :)